Polylactic acid or polylactide (PLA) is a polymer having optical properties and is currently used as a biodegradable medical material such as surgical suture and injection drug capsule. Recently, polylactic acid or polylactide (PLA) can be evaluated as an environmentally-friendly material as well as a biodegradable plastic which can be utilized for producing various kinds of polymer products, such as packaging materials, electric appliances, office supplies, and vehicle upholsteries.
To utilize polylactic acid for the above-mentioned applications, it is required for the polylactic acid to have high optical purity (D-form or L-form optical isomer) and a high molecular weight. For this purpose, lactide used as a monomer for producing the polylactic acid needs to have high optical and chemical purity.
Lactide is a kind of cyclic ester, which is a dimer generated by a dehydration reaction of lactic acid (or by a dealcoholization reaction of an ester compound of lactic acid).
A conventional method for producing lactide has involved the steps of producing a prepolymer having a molecular weight of about 100 to 5,000 by firstly polymerizing lactic acid or ester of lactic acid and then obtaining lactide by depolymerizing the prepolymer in the presence of a catalyst such as a metal oxide-based or tin-based catalyst under a reduced pressure condition with flowing inert gas. For example, U.S. Pat. No. 5,053,522 discloses a technique for producing an optically pure L(−)-lactide or D(+)-lactide by using the above-mentioned method, and U.S. Pat. No. 5,142,023 discloses a method for producing polylactide bioplastics from lactic acid, which includes removing water and a solvent to concentrate the lactic acid since the lactic acid will have to be polymerized to a prepolymer having a low-molecular-weight during the polymerization process due to its particular physical properties when moisture remains. Further, also described in U.S. Pat. No. 5,274,073 is a method for converting crude lactide to a purified lactide or polylactide, including the steps of obtaining a prepolymer by polymerizing lactic acid, then obtaining crude lactide by depolymerizing the prepolymer in the presence of a catalyst, and purifying the crude lactide with a distillation system. Besides, U.S. Pat. No. 5,247,059 discloses a process for producing a sufficiently purified lactide or polylactide from an ester of lactic acid, the process including the steps of producing a polylactide having a low molecular weight by the condensation of the ester of lactic acid using a polymerization catalyst, producing crude lactide by depolymerizing the polylactide having a low molecular weight and purifying the crude lactide. Further, U.S. Pat. No. 5,274,127 discloses a method for producing L-lactide, including the step of dehydrating an aqueous solution of L-lactic acid so that an average degree of polymerization does not exceed 2, and U.S. Pat. No. 6,277,951 describes a method for producing lactide through the steps of producing a polylactide having a low molecular weight from a purified liquid of lactic acid which is obtained by removing water and a solvent therefrom and synthesizing lactide by adding a catalyst to the polylactide having a low molecular weight. Moreover, U.S. Pat. No. 6,326,458 discloses a method of producing purified lactide or polylactide from lactic acid or ester of lactic acid, which includes the step of generating crude polylactide, wherein a catalyst is added to facilitate the generation of the lactide when the ester of lactic acid is used. However, the aforementioned production methods have drawbacks in that the depolymerization speed is slow, deterioration in optical purity occurs due to a high-temperature treatment, and there is difficulty in selecting the form and material of a reactor. Further, the lactides generated by the above-mentioned methods need to be subjected to an additional purifying process by recrystallization or distillation, which results in problems that the processes are complicated and excessive energy is required.
Meanwhile, U.S. Pat. No. 5,319,107 pertains to a method for producing cyclic ester such as lactide or glycolide from hydroxy carboxylic acid or its derivatives, and U.S. Pat. No. 5,420,304 describes a method of generating cyclic ester such as lactide by recovering lactic acid from a dilute lactic acid-containing solution by a solvent extraction method and converting the lactic acid to cyclic ester such as lactide while controlling a production proportion of oligomers having a high molecular weight during the dehydration process. Furthermore, disclosed in Japanese Patent Laid-open Publication No. 1993-286966 is a method for producing lactide having a low hygroscopic property from butyl ester of lactic acid. Besides, Japanese Patent Laid-open Publication No. 1999-209370 discloses a method for producing polylactide or lactide along with monobutyl tin compound by heating and then dealcoholizing an ester of lactic acid in the presence of monobutyl tin compound catalyst. Moreover, Japanese Patent Laid-open Publication No. 2001-181273 discloses a method for producing a polymer of α-hydroxy organic acid having a high molecular weight, including the steps of producing a higher alcohol ester of α-hydroxy organic acid oligomer by the dealcoholization and condensation of an α-hydroxy organic acid ester and a higher alcohol and removing glycolides by heating the higher alcohol ester thus obtained under a reduced pressure.
Further, U.S. Pat. No. 6,875,839 discloses a method for producing polylactide, including the steps of obtaining lactic acid from a starchy agricultural product by fermentation, purifying the lactic acid by ultrafiltration, nanofiltration, and/or electrodialysis, providing a prepolymer and performing a depolymerization process to obtain a dilactide. Moreover, U.S. Pat. No. 6,569,989 discloses a method for producing lactide, including the steps of synthesizing ester of lactic acid from ammonium lactate (ammonium salt of lactic acid) obtained by fermentation, performing a condensation-polymerization of the ester of lactic acid in the presence of a catalyst and depolymerizing the polylactide. In addition, disclosed in Japanese Patent Laid-open Publication No. 1995-304763 is another method for producing lactide, including the steps of producing oligomers of lactic acid from a mixture of lactic acid and ester of lactic acid, and heating and polymerizing the oligomers of lactic acid. Though the above-mentioned various methods have advantages in common in that a process load in the depolymerization step can be reduced by producing, in the prepolymerization step, the oligomers having a high content of lactyl lactate which is a linear dimer of the lactic acid or ester of lactic acid, these methods are not deemed to improve the conventional methods in view of the applied process steps and necessity for a depolymerization process.
Further, U.S. Pat. No. 5,332,839 pertains to a method for directly converting lactide from a lactic acid-containing solution, including direct condensation of lactic acid or oligomer of lactic acid (having a degree of polymerization equal to or less than about 4) in the presence of a fixed-bed catalyst, and Japanese Patent Laid-open Publication No. 1999-092475 relates to a method involving the 2 molecule condensation-cyclization of lactic acid and/or ammonium salt of lactic acid and recovering lactide from a distillation tower. However, these methods have a defect in that deactivation problem of catalysts is serious, making it difficult to carry out a continuous process. Further, as for the technique of recovering only the lactide from the distillation tower, a production yield of lactide is low, and it has been difficult to obtain high-purity lactide from non-reacted lactic acid or oligomer of lactic acid (linear dimer or trimer of lactic acid, or the like).
Meanwhile, though high-purity lactic acid is required as a raw material to obtain a lactide monomer having purity for polymerization, it is difficult to purify the lactic acid because the lactic acid readily forms oligomer in a concentration process. Accordingly, to obtain a high-purity lactic acid from fermented liquid of lactic acid, it is general to pre-treat the fermented liquid, convert it to an ester compound of lactic acid, and purify the ester compound of lactic acid followed by hydrolysis thereof, to thereby obtain high-purity lactic acid (Separation and Purification Technology 52, (2006), 1-17). Accordingly, compared to obtaining lactide by pre-polymerizing the high-purity lactic acid obtained by the above-described method and again depolymerizing the obtained pre-polymer, the direct conversion of the ester compound of lactic acid to the lactide is deemed to be a more efficient way to simplify the process and reduce energy consumption greatly.
As a known technology to directly convert ester of lactic acid to lactide, disclosed in Japanese Patent Laid-open Publication Nos. 1999-036366 and 1993-286966 is a method for the condensation of ester of lactic acid containing an alkyl group having 1 to 8 carbon atoms with heating under a reduced pressure condition using a catalyst such as an organic tin compound such as mono-butyl tin oxide, dibutyl tin oxide, dibutyl tin lactate, stannous octonate or the like, zinc chloride (ZnCl2), tin chloride (SnCl2), calcium chloride (CaCl2), phosphoric acid (H2PO4), p-toluenesulfonic acid, or the like. Furthermore, Japanese Patent Laid-open Publication No. 1994-031175 discloses a method for producing lactide by performing a 2-molecule condensation-cyclization of ester of lactic acid containing a lower alkyl group having 1 to 6 carbon atoms in the presence of dibutyl tin chloride catalyst or a catalyst made of a combination of dibutyl dichloro tin, and phosphorus pentoxide or phosphorus trioxide.
However, the above-stated method using the organic tin catalyst is disadvantageous in that a production proportion of Meso-form optical isomer is high while a yield of lactide is low.
In view of the foregoing, the present inventors have conducted many researches to develop a method for directly converting lactide from ester of lactic acid with a high yield of lactide while maintaining optical property (D-form or L-form optical isomer), and finally reached the present invention which relates to a method for producing lactide directly from ester of lactic acid using a titanium-based catalyst, which shows a high yield while maintaining optical property (D-form or L-form optical isomer).